Method of growing electrical conductors

ABSTRACT

A method for forming a conductive thin film includes depositing a metal oxide thin film on a substrate by an atomic layer deposition (ALD) process. The method further includes at least partially reducing the metal oxide thin film by exposing the metal oxide thin film to a gaseous inorganic reducing agent, thereby forming a metal layer. In preferred arrangements, the reducing agent comprises of thermal hydrogen (H 2 ), hydrogen radicals (H*) and/or carbon monoxide (CO).

PRIORITY APPLICATION

This application claims the benefit of U.S. Provisional Application60/492,486, filed Aug. 4, 2003, the entire disclosure of which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to the manufacturing ofintegrated circuits (ICs), and particularly to thin metal layers, suchas for seed layers in damascene and dual damascene processes, gatemetals of thin film transistors and capacitor electrodes in ICs.

DESCRIPTION OF THE RELATED ART

The atomic layer deposition (ALD) method of depositing thin films hasseveral attractive features including excellent step coverage, even onlarge areas, and a dense and pinhole-free structure. Therefore, it is ofgreat interest to apply ALD to the deposition of metallization layers ofadvanced integrated circuits (ICs), where the continuously increasingpacking density and aspect ratio set higher and higher demands upon themetallization layers. Applications where high quality metallization isparticularly useful include dual damascene structures, gates intransistors and capacitors in ICs. However, due to the fact that ALD isbased on sequential self-saturating surface reactions of source chemicalcompounds and utilization of active surface sites, depositing highquality elemental metal thin films by ALD is very difficult.

In ALD, the source chemical molecules chemisorb on the substrate viaactive sites on the substrate surface. Typical active sites for metalsource chemicals are —OH, —NH₂ and —NH groups. Metal-oxygen-metalbridges on the surface may also act as active sites. When a metal sourcechemical molecule reacts with the active site, a strong bond is formedbetween the surface and source chemical molecule and the ligand of thesource chemical molecule is simultaneously released as a by-product.

In ALD, films grow with a constant growth rate. Each deposition cycleproduces about one molecular layer of the deposited material on thesubstrate surface. Usually the growth rate is well below one molecularlayer/cycle because the adsorbed source chemical molecules may be bulky(steric hindrance) or because substrate temperature affects the numberof active sites (e.g., OH groups) on the surface. It is well known thatmetal oxide thin films produced by ALD are uniform, have excellentadhesion and thus are firmly bonded to the substrate surface.

In contrast to metal oxide films, experiments have revealed a drawbackof the growth of metal thin films by an ALD type method. In the case ofmetal deposition it is difficult to attach source chemical molecules tothe surface because few active sites exist on the surface. The metalfilm grown is often non-uniform over an area of the substrate and it iseasily peeled off from the surface, which indicates very poor adhesionof the film to the substrate.

Several attempts have been made to produce metal thin films by ALD typemethods. Reproducibility of such an ALD metal growth process hastraditionally been poor and the reactions do not take place at all oninsulating surfaces like silicon oxide. There are publications about theALD deposition of Cu metal by pulsing a copper compound, e.g., Cu(thd)₂,on a surface and then reducing the Cu(thd)₂ molecules bound to thesurface into Cu with H₂.

R. Solanki et al. (Electrochemical and Solid-State Letters 3 (2000)479-480) have deposited copper seed layers by ALD. They deposited copperdirectly from alternate pulses ofbis(1,1,1,5,5,5-hexafluoroacetylacetonato)copper(II)hydrate and eithermethanol, ethanol or formalin, i.e., a water solution of formaldehyde.The total pulsing cycle time was 64 s, i.e., slightly over one minute.Although the growth rate was not mentioned in the publication, a typicalgrowth rate of a thin film made by ALD from metal P-diketonates is 0.03nm/cycle due to the steric hindrance of the source chemical molecules.Thus, the deposition time for a 10 nm copper seed layer would be over 5hours, which is uneconomical for wafer processing. A commerciallyacceptable minimum throughput of a wafer reactor is 10-12 wafers/hour.It is to be noted that according to Strem Chemicals, Inc. thedecomposition temperature of the copper compound used by R. Solanki etal. is 220° C. R. Solanki et al. noticed copper film growth when thesubstrate temperature was 230-300° C. Therefore, partial thermaldecomposition of copper source compound (contrary to ALD self-limitingsurface reaction principles) on substrate surface is probable.

One of the most advanced IC structures is the dual damascene structurefor interconnecting integrated circuit devices such as transistors(which include source, gate and drain). Several electrically conductinglayers are needed in the structure. The first metallization level isdone with tungsten plugs and aluminum interconnects to prevent thecontamination of the gate with copper. The remainder of themetallization levels are made of copper in state-of-the-art ICs, tomaximize conductivity and thus circuit speed.

There are several ways of making dual damascene structures. An exampleof the process steps of a dual damascene process is described below.

Step 1. A silicon nitride etch stop is grown on the previousmetallization surface.

Step 2. A via level dielectric is deposited.

Step 3. Another silicon nitride etch stop is deposited.

Step 4. A trench level dielectric is deposited. SiO₂ has been favored asthe dielectric material and the examples below assume some form of SiO₂for the trench- and via-level dielectrics. Low-k materials such ascarbon-doped silicon oxide and polymers have been experimented with asalternative dielectric materials.

Step 5. Patterning of the dielectric layers by photolithography.

-   -   a. A resist layer is deposited on dielectric stack's surface.    -   b. The resist layer is patterned and the resist is removed from        the via areas.    -   c. The level dielectric is etched from the via areas with        directional plasma etching, terminating at the upper silicon        nitride etch stop surface.    -   d. Resist is stripped from the surface.

Step 6. Patterning of the etch stop layer by photolithography.

-   -   a. A second resist layer is deposited on the surface.    -   b. The resist layer is patterned and it is removed from the        trench areas.    -   c. The upper silicon nitride is removed with a short plasma        nitride etch from the bottom of the holes that were made with        the first plasma oxide etch, creating a buried hard mask.    -   d. The second plasma oxide etch removes silicon dioxide from the        exposed via and trench areas until the silicon nitride etch stop        layers are reached.    -   e. The lower silicon nitride etch stop is removed from the via        bottom and the upper silicon nitride etch stop from the trench        bottom with a short plasma nitride etch.    -   f. The resist is stripped from the substrate.

Step 7. The surface of the substrate is cleaned.

Step 8. A diffusion barrier layer is grown on all exposed surfaces.

Step 9. A seed layer for copper deposition is grown with CVD or PVD onthe diffusion barrier layer.

Step 10. Vias and trenches are filled with bulk copper, such as by anelectroplating process.

Step 11. The substrate surface is planarized with chemical mechanicalpolishing (CMP). The surface is polished until copper and a barrierlayer are left only in trenches and vias.

Step 12. The surface is capped with a silicon nitride etch stop layer.

Step 13. The metallization process is then repeated for all theremaining metallization levels.

Alternatives for copper electroplating (Step 10) are electrolessplating, physical vapor deposition (PVD), metal organic chemical vapordeposition (MOCVD) and copper superfill that is based on a catalyzedMOCVD process. A seed layer (c.f. Step 9) is typically needed forelectroplating processes. The seed layer is advantageous also for CVDprocesses because it can act as a nucleation layer for the thin filmdeposition by the CVD processes. Traditionally such a seed layer isdeposited by chemical vapor deposition (CVD) or physical vapordeposition (PVD). In the electroplating process the substrate having anelectrically conductive seed layer is immersed in a metal compoundsolution. The electrically conductive surface of the substrate isconnected to an external DC power supply. A current passes through thesubstrate surface into the solution and metal is deposited on thesubstrate. The seed layer has high electrical conductivity and it actsas a current conduction and nucleation layer for the electroplatingprocess. The seed layer carries current from the edge of the wafer tothe center of the wafer and from the top surface of the wafer into thebottom of vias and trenches. A uniform and continuous seed layer isnecessary to get uniform electroplated copper. Electrical contact ismade to the seed layer. The quantity of the deposited metal is directlyproportional to the local current density on the substrate.

The benefits of copper compared to aluminum are lower resistivity andbetter resistance to electromigration. Furthermore, since tighterpacking density can be obtained with copper, fewer metallization levelsare needed and the manufacturing costs are lower than with aluminum.With increasing aspect ratio it is becoming difficult to get sufficientstep coverage for the seed layer with the state of the art technology.

In dynamic random access memories (DRAM), capacitors store data bits inthe form of electrical charge. These memory capacitors must be rechargedfrequently due to charge leakage. The simplest capacitor consists of twoparallel metallic plates separated with a dielectric material. Thecapacity (C) of this plate capacitor depends according to equation (I)on the area (A) of the metallic plate, the distance (d) between themetallic plates and the dielectric constant (k) of the dielectricmaterial. ε₀ is the permittivity of space.C=kε ₀ A/d  (I)

Cylindrical capacitors are often used. The conductors are arrangedcoaxially. The charge resides on the inner surface of the outerconductor or on both the inner surface of the outer conductor and on theouter surface of the inner conductor. In this case the capacitance (C)depends on the radius of the outer surface of the inner conductor (a),radius of the inner surface of the outer conductor (b), length of thecylinder (1) and dielectric constant (k) of the dielectric materialbetween the conductors as shown in equation (II).C=2πkε₀ l/ln(b/a)  (II)

The feature sizes in DRAMs are decreasing continuously. The capacitorsmust be made smaller in successive DRAM generation. In order to savesurface area, planar capacitors are being replaced with vertical coaxialcapacitors that may have aggressive aspect ratios. However, with scalingthe footprint available for each capacitor is reduced. For a givencapacitor size, the charge storing area is reduced, such that thedistance between the conductors must be decreased and/or the dielectricconstant of the dielectric must be increased in order to keep thecapacity sufficient. Decreasing the distance between the conductorscauses voltage breakdown when the insulator thickness is too thin tohold the voltage.

Using high-k dielectrics, such as TiO₂ and Ta₂O₅, resolves theabove-described problem related to decreasing feature size. However, theaforementioned high-k dielectrics create new problems, since they donateoxygen to the conductor and thus the capacitor properties deteriorate.Therefore, inert metals, such as platinum group metals, or conductivemetal oxides, such as RuO₂, are favored for the electrode surfacesadjacent to the high-k metal oxides. But it is difficult to deposit thinfilms with good step coverage on new capacitor structures with smallfeature size and aggressive aspect ratio. As a conclusion, there is anincreasing need for a method of producing conductive thin films withgood step coverage and excellent thin film properties, such as adhesionto the substrate.

S.-J. Won et al. have presented a metal-insulator-metal (MIM) capacitorstructure for giga-bit DRAMs (Technical Digest of the 2000 InternationalElectron Devices Meeting (IEDM), San Francisco, Calif., Dec. 10-13,2000). They used Ta₂O₅ as the capacitor dielectric while the electrodesconsisted of ruthenium deposited by CVD from Ru(EtCp)₂ and gaseousoxygen at 300-400° C. Problems related to the method included poor stepcoverage and reaction speed sensitivity. When the nodes were made with0.08 μm design rules, the step coverage dropped to 60%. The reaction ofRu(EtCp)₂ with O₂ was adversely affected by the partial pressures of thecompounds.

N⁺ or p⁺ doped polycrystalline silicon (poly-Si) has been used as a gateelectrode for transistors. However, several problems are associated withthe use of poly-Si gate electrodes. In the case of boron-doped p⁺poly-Si, the diffusion of boron through the gate SiO₂ destroys theelectrical properties of the transistor. Poly-Si is thermodynamicallyunstable against high dielectric constant materials at high processingtemperatures. In addition, poly-Si has rather high resistivity comparedto metals. There is a tendency to replace the SiO₂ gate oxide with ahigh dielectric constant metal oxide. A metal with appropriate workfunction would enable the tailoring of the CMOS threshold voltage.Refractory metals have been suggested for gate metals but the stabilityof the metal—gate oxide interface has been an issue. Platinum groupmetals are potential candidates for gate metals due to their inertnature. However, appropriate methods of depositing high-quality platinumgroup metal thin films for gate electrode applications have not yet beendeveloped.

M. Utriainen et al. have demonstrated (Appl. Surf. Sci. 157 (2000) pp.151-158) that ALD grown metal oxides can be used as interconnects in ICsafter reducing the metal oxides into metals. They studied the direct ALDdeposition of Cu, Ni and Pt metals and the indirect Ni metal growthmethod via reduction of NiO. However, they had problems with the qualityof the nickel film: pinholes were formed on the thin films during thereduction of NiO with hydrogen gas.

SUMMARY OF THE INVENTION

Embodiments described herein provide methods of producing high qualityconductive thin films with excellent step coverage, uniform thicknessover a large area and excellent adhesion properties. The thin films maybe used, for example, as seed layers for the electrodeposition of metallayers, as gate metals in thin film transistors and as capacitorelectrodes for advanced high-density integrated circuits.

The present method is particularly applicable to the manufacture ofconductive thin films, preferably comprising one or more of thefollowing elements: rhenium, ruthenium, osmium, cobalt, rhodium,iridium, nickel, palladium, platinum, copper, silver and gold.

According to one embodiment described herein, a method of forming aconductive thin film comprises depositing a metal oxide thin film on asubstrate by an atomic layer deposition (ALD) process. The methodfurther comprises at least partially reducing the metal oxide thin filmby exposing the metal oxide thin film to a gaseous inorganic precursor,thereby forming a seed layer. In certain embodiments, the reduction ofthe metal oxide thin film essentially converts the metal oxide into anelemental metal seed layer that has sufficient electrical conductivityto be used for subsequent electrochemical deposition. In otherembodiments, the reduction of the metal oxide thin film essentiallyconverts the metal oxide into an elemental metal seed layer that can beused as a nucleation layer for the subsequent deposition of bulk metalby CVD and MOCVD processes.

According to another embodiment described herein, a method of producinga conductive thin film comprises the steps of (A) placing a substrate ina chamber and (B) exposing the substrate to a vapor phase firstreactant. The first reactant adsorbs no more than a monolayer of metalspecies on the substrate. The method further comprises (C) removingexcess first reactant from the chamber and (D) exposing the substrate toa second vapor phase reactant comprising a compound that is capable ofoxidizing the adsorbed metal species on the substrate into metal oxide.The method further comprises (E) removing excess second reactant fromthe chamber and (F) repeating the above steps B through E at least threetimes to form a metal oxide film of desired thickness. The methodfurther comprises (G) following step F, exposing the substrate to agaseous inorganic precursor to reduce the metal oxide film to a metalfilm.

According to still another embodiment described herein, a method ofproducing a conductive thin film comprises depositing a metal oxide thinfilm of at least 0.6 nm thickness on a substrate. The method furthercomprises reducing said metal oxide thin film to metal thin film byexposing the metal oxide thin film to a gaseous inorganic precursor.

According to yet another embodiment described herein, a method ofproducing a conductive thin film comprises depositing a metal oxide thinfilm on a substrate by an atomic layer deposition (ALD) process. Themethod further comprises at least partially reducing the metal oxidethin film to an elemental metal film. In certain embodiments, the metaloxide thin film is at least partially reduced by exposing the metaloxide thin film to a gaseous inorganic precursor.

According to another embodiment described herein, a method of producinga conductive thin film comprises depositing a metal oxide thin film on asubstrate by an atomic layer deposition (ALD) process. The methodfurther comprises at least partially reducing the metal oxide thin filmto elemental metal film in a CVD tool.

Certain embodiments described herein are especially beneficial formaking electrically conductive layers in structures that have highaspect ratios, like vias, trenches, local high elevation areas and othersimilar surface structures that make the surface rough and complicatethin film processing by conventional CVD and PVD methods. An ALD metaloxide process combined with a reduction step provides excellent stepcoverage of electrically conductive thin films on all surfaceformations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a dual damascene structure.

FIG. 2 is a schematic view of a capacitor structure.

FIG. 3 is a schematic view of an NMOS transistor suitable for CMOSstructures.

FIG. 4 depicts a general sequence of processes.

FIG. 5 schematically illustrates a first cluster tool in accordance withembodiments of the present invention.

FIG. 6 schematically illustrates a second cluster tool in accordancewith embodiments of the present invention.

FIG. 7 depicts sheet resistance versus reduction time for 1000 and 1500cycles of Cu_(x)O reduced with different H₂ concentrations at 1500 W,560 Pa and 150° C.

FIG. 8 depicts sheet resistance versus H₂ reduction concentration for500 cycles of Cu_(x)O reduced at 1500 W, 560 Pa and 150° C.

FIG. 9 depicts sheet resistance versus reduction time for 1000 cycles ifCu_(x)O reduced at 250 W, 560 Pa and at different temperatures.

FIG. 10 depicts sheet resistance versus reduction time for differentnumbers of cycles of Cu_(x)O reduced at 250 W, 560 Pa and 150° C.

DETAILED DESCRIPTION OF THE DRAWINGS

The dual damascene structure 100 shown in FIG. 1 includes an underlyingmetallization layer 102 (e.g., Cu), an insulating layer 104 (e.g.,SiO₂), a via etch stop 106 (e.g., Si₃N₄), a via level insulator 108(e.g., SiO₂), a trench etch stop 110 (e.g., Si₃N₄), a trench levelinsulator 112 (e.g., SiO₂), a diffusion barrier 114 (e.g., TaN), a seedlayer 116 (not shown to scale) and a via/trench fill metal 118 (bulkmetal, e.g., Cu).

The capacitor structure 200 shown in FIG. 2 includes a contact plug 202(e.g., tungsten (W) or polysilicon), an insulator 204, an optionaldiffusion barrier 206 (e.g., TiN), a lower electrode 208 (e.g., Ru, Pt,or RuO₂), a high-k dielectric film 210 (e.g., barium strontium titanate(BST)), and an upper electrode 212 (e.g., Ru or Pt).

The partial transistor structure 300 shown in FIG. 3 includes asubstrate 302, an n-type well 304, a p-type diffusion region 306 (rightdrain, left source), a shallow trench isolation oxide 308, a gatedielectric 310, an optional barrier layer 312, a gate metal 314, gateisolation spacers 316, and contact areas for tungsten plugs 318. Thecontact areas are shown dotted because they are not in the same verticalplane with the other numbered parts. A CMOS structure contains both PMOSand NMOS transistors. The contact areas against P-type semiconductor canbe made of, e.g., Ni, or RuO. The contact areas against N-typesemiconductor can be made of, e.g., Ru. Platinum can also be appliedunder W plugs. The choice of the metal or electrically conductive metalcompound depends on the work function of the underlying layer and thereactivity of the surrounding materials with the metal or electricallyconductive metal compound.

A typical process sequence shown in FIG. 4 consists of cleaning of asubstrate 400, deposition of a diffusion barrier 402, deposition of ametal oxide layer 404, reduction of the metal oxide layer into a metalseed layer 406 and deposition of bulk metal 408.

FIG. 5 schematically illustrates a first cluster tool 500 that consistsof load locks 510, 550, vacuum transport module 520, a first reactionchamber 530 and a second reaction chamber 540.

FIG. 6 schematically illustrates a second cluster tool 600 that consistsof load locks 610, 660, a vacuum transport module 620, a first reactionchamber 630, a second reaction chamber 640 and a third reaction chamber650.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A layer of a low volatility oxide of a metal is grown on a substrate.Preferably the metal oxide is grown on the substrate according to theprinciples of an ALD process, and the following disclosure is directedto this embodiment. However, the skilled artisan will recognize thatother methods of depositing a metal oxide thin film may be used in themethods. After the ALD process, the thin film consists essentially of ametal oxide or a mixture of metal oxides. The metal oxides are at leastpartially converted into a metal in a separate process step to increasethe electrical conductivity of the deposited oxide thin film. Theconversion step can be done with any inorganic reducing agent capable offorming a stronger bond to oxygen than the metal to be reduced.Preferably, the reducing agent is in the gaseous phase. However, in thecase of silver and gold oxides, the conversion step can also be donesimply by heating to decompose the oxide into metal and oxygen.

The following low volatility metal oxides and mixtures and nanolaminatesof the following metal oxides are examples of compounds that aresuitable for conversion into a conductive form by the method of certainembodiments: ReO₂, Re₂O₅, ReO₃, RuO₂, OSO₂, CoO, CO₃O₄, Rh₂O₃, RhO₂,IrO₂, NiO, PdO, PtO₂, Cu₂O, CuO, Ag₂O (decomposes at temperatures aboveabout 230° C.), Au₂O₃ (decomposes at temperatures above about 160° C.).However, a person skilled in the art will understand that embodimentsdescribed herein are not limited to these metal oxides, in part becausethe stoichiometry may vary in metal oxide films. In addition, thefollowing high-volatility metal oxides exist: Re₂O₇, RuO₄ and OSO₄.

A metal oxide layer is preferably produced by an ALD process. A typicalALD process comprises the following steps:

-   -   1. placing a substrate into a reaction chamber;    -   2. adjusting the pressure and temperature of the reaction        chamber, before or after placement of the substrate;    -   3. feeding into the reaction chamber and contacting the        substrate with a pulse of at least one first source chemical,        preferably in the vapor phase, comprising a compound capable of        adsorbing no more than a monolayer of metal species on the        substrate;    -   4. removing gases from the chamber (e.g., by purging);    -   5. feeding into the reaction chamber and contacting the        substrate with a pulse of at least one second source chemical,        preferably in the vapor phase, comprising a compound capable of        oxidizing the metal species on the substrate into a metal oxide;    -   6. removing gases from the chamber (e.g., by purging); and    -   7. repeating steps 3 through 6 until a desired thickness of the        growing thin film is reached.

According to the ALD principles, the previous reactant (i.e., previouslypulsed source chemical) and the gaseous by-products of the surfacereaction are removed from the reaction chamber before the next pulse ofa reactant is introduced into the reaction chamber. The reactants andthe by-products can be removed from the reaction chamber by pumping downthe chamber to a higher vacuum by a vacuum pump, by purging the chamberwith an inert gas pulse, or by a combination of the two. In certainembodiments an inert gas is continuously flowing through the reactionspace and one reactant at a time is injected in gaseous form to theflowing inert gas. The injections of different reactants are separatedin time from each other so that there is only one reactant present inthe gas phase of the reaction space at a time.

In the methods of certain embodiments described herein, the ALD cycledescribed above is preferably repeated at least 3 times, more preferablyat least 10 times prior to reduction. A metal oxide thin film of atleast about 0.6 nm is preferably formed on the substrate before it is atleast partially reduced with the inorganic reducing agent.

“Metal species” in the context of the present application means aseparate molecule, atom or ion comprising one or more metal atoms.

According to one embodiment (FIG. 1), a substrate with open trenches andvias is provided into an ALD reaction chamber. A diffusion barrier layer114 (e.g., WNC) is provided on the surfaces of the substrate. Thepressure of the reaction chamber is adjusted to about 5-10 mbar with avacuum pump and flowing nitrogen gas. A metal oxide thin film is grownon the diffusion barrier 114 by ALD from alternate pulses of a metalsource chemical and oxygen source chemical. Surplus source chemical andreaction by-products are removed essentially entirely from the reactionchamber after each source chemical pulse before the next source chemicalpulse is introduced into the reaction chamber and contacted with thesubstrate surface. The pulsing cycle is repeated, preferably until thethickness of the metal oxide film is sufficient to produce a metal filmfor seed layer purposes. The metal oxide film is subsequently reducedinto a metal layer and used as a seed layer 116 for an electroplatingprocess, an MOCVD process or a copper superfill process.

According to a second embodiment (FIG. 2), a substrate is provided intoa reaction chamber of an ALD reactor. The substrate is heated to adeposition temperature of selected metal oxide. Alternate gas phasepulses of a metal source chemical and an oxygen source chemical areintroduced into the reaction chamber and contacted with the substratesurface. A metal oxide film is thus grown by ALD on the surface. Themetal oxide is used as the first electrode 208 of a capacitor orconverted into corresponding metal and used as the first electrode 208of the capacitor. Then a thin film of a high-k dielectric material 210is grown on the first electrode 208. The high-k layer 210 is optionallyannealed. A second metal oxide thin film is grown by ALD on the high-klayer. The metal oxide film is converted into corresponding metal andused as the second electrode 212 of a capacitor. However, the metaloxide thin film can be used as the second electrode of the capacitor ifthe conductivity of the metal oxide thin film is sufficiently high.

In certain embodiments, the metal oxide thin film is used as the secondelectrode when its resistivity is preferably less than about 500 μΩ-cm,more preferably less than about 300 μl-cm, and most preferably less thanabout 100 μΩ-cm. An example of a suitable metal oxide is rutheniumdioxide (RuO₂), which has a resistivity of about 35 μΩ-cm.

According to a third embodiment (FIG. 3), a substrate is provided into areaction chamber of an ALD reactor. The surface may be, for example, atransistor gate oxide 310 or doped silicon in source and drain areas306. The substrate is heated to a deposition temperature. Alternate gasphase pulses of a metal source chemical and an oxygen source chemicalare introduced into the reaction chamber and contacted with thesubstrate surface. Metal oxide film is thus grown by ALD on the surface.The metal oxide is used as the gate electrode 314 of a transistor assuch or converted into the corresponding metal and used as the gateelectrode 314 of a transistor. The metal is also used as an intermediatelayer 318 between silicon and tungsten plugs on the source and the drainareas 306 of the transistor.

When depositing silver and gold oxides by ALD, special attention is paidto the selection of growth temperatures, since Ag₂O decomposes into Agand O₂ at about 230° C. and Au₂O₃ decomposes into Au and O₂ at about160° C. Therefore, the deposition temperature of silver oxide ispreferably kept below 230° C. and the deposition temperature of goldoxide is preferably below 160° C.

The Source Chemicals

The ALD source chemicals must have sufficient volatility at the sourcetemperature to saturate the substrate surface. The vapor pressure of thesource chemical should be at least about 0.02 mbar at the sourcetemperature to enable reasonably short pulse times for saturating thesubstrate surfaces. The metal source chemicals should also be thermallystable at the deposition temperature to prevent particle formation inthe gas-phase of the reaction chamber.

Suitable metal source compounds are sold, for example, by StremChemicals, Inc. (7 Mulliken Way, Dexter Industrial Park, Newburyport,Mass., USA) and Tri Chemical Laboratory, Inc. (969 Helena Drive,Sunnyvale, Calif., USA).

Low oxidation state rhenium oxide can be grown by ALD for example fromfollowing rhenium compounds:

-   rhenium(VII)oxide (Re₂O₇), rhenium pentacarbonyl chloride    (Re(CO)₅Cl), methyltrioxorhenium(VII) (CH₃ReO₃),    cyclopentadienylrhenium tricarbonyl ((C₅H₅)Re(CO)₃),    pentamethylcyclopentadienylrhenium tricarbonyl ([(CH₃)₅C₅]Re(CO)₃),    and i-propylcyclopentadienylrhenium tricarbonyl ((C₃H₇)C₅H₄Re(CO)₃).

Low oxidation state ruthenium oxide can be grown by ALD for example fromfollowing ruthenium compounds:

-   ruthenium(VIII)oxide (RuO₄), bis(cyclopentadienyl)ruthenium    ((C₅H₅)₂Ru), bis(pentamethylcyclopentadienyl)ruthenium ([(CH₃)₅C₅]    ₂Ru), cyclopentadienylruthenium dicarbonyl ((C₅H₅)₂Ru(CO)₂),    bis(2,2,6,6-tetramethyl-3,5-heptanedionato)(1,5-cyclooctadiene)ruthenium(II)    (C₁₁H₁₉O₂)₂(C₈H₁₂)Ru, tris(dipivaloylmethanato)ruthenium i.e.    tris(2,2,6,6-tetramethyl-3,5-heptanedionato)ruthenium (Ru(DPM)₃ or    Ru(thd)₃), anhydrous rutheniumnitrate (Ru(NO₃)₃), and anhydrous    ruthenium (III) nitrosyl nitrate (Ru(NO)(NO₃)₃).

Low oxidation state osmium oxide is preferably grown by ALD for examplefrom following osmium compounds: bis(cyclopentadienyl)osmium((C₅H₅)₂Os), bis(pentamethylcyclopentadienyl)osmium ([(CH₃)₅C₅]₂Os), andosmium(VIII)oxide (OSO₄).

Cobalt oxide is preferably grown by ALD for example from followingcobalt compounds:

-   bis(cyclopentadienyl)cobalt(II) ((C₅H₅)₂Co),    bis(methylcyclopentadienyl)cobalt(II) ((CH₃C₅H₄)₂Co),    bis(pentamethylcyclopentadienyl)cobalt(II) ([(CH)₃C₅]₂Co), cobalt    tricarbonyl nitrosyl (Co(CO)₃NO), cyclopentadienylcobalt dicarbonyl    C₅H₅CO(CO)₂, cobalt(III)acetylacetonate (Co(CH₃COCHCOCH₃)₃),    tris(2,2,6,6-tetramethyl-3,5-heptanedionato)cobalt(III) i.e.    tris(dipivaloylmethanato)cobalt (Co(TMHD)₃, or Co(DPM)₃, or    Co(thd)₃, or Co(C₁₁H₁₉O₂)₃), and anhydrous cobalt nitrate    (Co(NO₃)₃), the synthesis of which has been described by R. J. Logan    et al. in J. Chem. Soc., Chem. Commun. (1968) 271.

Rhodium oxide is preferably grown by ALD for example from followingrhodium compounds:

-   2,4-pentanedionatorhodium(I)dicarbonyl (C₅H₇Rh(CO)₂),    tris(2,4-pentanedionato)rhodium i.e. rhodium(III)acetylacetonate    (Rh(C₅H₇O₂)₃), and tris(trifluoro-2,4-pentanedionato)rhodium.

Iridium oxide is preferably grown by ALD for example from followingiridium compounds:

-   (methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(I)    ([(CH₃)C₅H₄](C₈H₁₂)Ir) and trisallyliridium ((C₃H₅)₃Ir).

Nickel oxide is preferably grown by ALD for example from followingnickel compounds:

-   nickel carbonyl (Ni(CO)₄),    bis(2,2,6,6-tetramethyl-3,5-heptanedionato)nickel(II) (Ni(DPM)₂, or    Ni(thd)₂, or Ni(C₁₁H₁₉O₂)₂), nickel(II)acetylacetonate, also known    as bis(2,4-pentanedionato)nickel(II),    nickel(II)trifluoroacetylacetonate,    nickel(II)hexafluoroacetylacetonate (Ni(CF₃COCHCOCF₃)₂),    nickel(II)dimethylglyoxime (Ni(HC₄H₆N₂O₂)₂), and    tetrakis(trifluorophosphine)nickel(O) (Ni(PF₃)₄).

Palladium oxide is preferably grown by ALD for example from followingpalladium compounds: Pd(thd)₂ andbis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)palladium(Pd(CF₃COCHCOCF₃)₂).

Platinum oxide is preferably grown by ALD for example from followingplatinum compounds:

-   platinum(II)hexafluoroacetylacetonate (Pt(CF₃COCHCOCF₃)₂),    (trimethyl)methylcyclopentadienylplatinum(IV) ((CH₃)₃(CH₃C₅H₄)Pt),    and allylcyclopentadienylplatinum ((C₃H₅)(C₅H₅)Pt).

Copper oxide is preferably grown by ALD for example from the followingcopper compounds and their derivatives: CuCl, CuBr, CuI,bis(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)copper(II)(Cu(FOD)₂), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II)(Cu(TMHD)₂, or Cu(DPM)₂, or Cu(thd)₂), copper(II)acetylacetonate(Cu(CH₃COCHCOCH₃)₂), also known as Cu(acac)₂, derivatives of Cu(acac)₂such as alkyl derivatives of Cu(acac)₂,copper(II)trifluoroacetylacetonate (Cu(CF₃COCHCOCH₃)₂),copper(II)hexafluoroacetylacetonate (Cu(CF₃COCHCOCF₃)₂),hexafluoroacetylacetonatocopper(I)trimethylphosphine adduct(Cu(CF₃COCHCOCF₃)P(CH₃)₃), copper(II)beta-ketoiminates,copper(II)beta-diketiminates such asbis(N,N′-Dialkyl-1,3-alkyldiketiminato)copper(II), e.g.bis(N,N′-Diethyl-1,3-propanediketiminato)copper(II),(N,N′-Dialkyl-2-alkyl-amidinato)copper(I) complexes such as(N,N′-Diisopropylacetamidinato)copper(I) dimer [Cu(^(i)Pr-MeAMD)]₂, thesynthesis of which has been described by B. S. Lim et al. (InorganicChemistry (2003) pp. 7951-7958, incorporated by reference herein),copper(II)dialkylaminoalkoxides such as copper(II)dimethylaminoethoxide,cyclopentadienylcopper(I)triethylphosphine ((C₅H₅)Cu:P(C₂H₅)₃),ethylcyclopentadienylcopper triphenylphosphine adduct((C₂H₅C₅H₄)Cu:P(C₆H₅)₃),hexafluoroacetylacetonatocopper(I)triethylphosphine adduct((C₅HF₆O₂)Cu:P(C₂H₅)₃), hexafluoroacetylacetonatocopper(I)₂-butyneadduct ((C₅HF₆O₂)Cu:CH₃C≡CCH₃), hexafluoroacetylacetonatocopper(I)1,5-cyclooctadiene adduct ((C₅HF₆O₂)Cu:C₈H₁₂),hexafluoropentanedionatocopper(I)vinyltrimethylsilane adduct, andanhydrous copper nitrate (Cu(NO₃)₂), the synthesis of which has beendescribed by C. C. Addison et al. (J. Chem. Soc. (1958) pp. 3099-3106,incorporated by reference herein).

Silver oxide is preferably grown by ALD for example fromhexafluoroacetylacetonatosilver trimethylphosphine adduct((C₅HF₆O₂)Ag:P(CH₃)₃).

Gold oxide is preferably grown by ALD for example from following goldcompounds:

-   gold(III)fluoride AuF₃, dimethyl(acetylacetonato)gold(III)    ((CH₃)₂(C₅H₇O₂)Au), and dimethylhexafluoroacetylacetonatogold    ((CH₃)₂Au(C₅HF₆O₂)).

The oxygen source material used in the method of certain embodiments isselected from a group of volatile or gaseous compounds that containoxygen and are capable of reacting with an adsorbed metal compound onthe substrate surface, at the deposition conditions, resulting in growthof metal oxide thin film on the substrate surface.

It is to be noted that Re, Ru and Os form highly volatile oxides whenreacting with strong oxidizing agents. It is therefore preferable toexclude strong oxidizing agents from the vicinity of the substrate whengrowing lower oxidation state oxides of Re, Ru and Os.

In the production of a metal oxide thin film on a wafer the oxygensource chemical is selected for example from a group consisting of water(H₂O), hydrogen peroxide (H₂O₂), oxygen (O₂), ozone (O₃), singlet oxygen102, and oxygen compounds with unpaired electrons, as well as oxygenradicals and OH radicals.

A special group of oxygen source chemicals can be used when the metalsource chemical is an anhydrous metal nitrate. This group of oxygenchemicals consists of an aqueous solution of ammonia (NH₃*H₂O or NH₄OH),an aqueous solution of hydroxylamine (NH₂OH*H₂O), and an aqueoussolution of hydrazine (N₂H_(4*)H₂O).

Mixtures of at least two oxygen source chemicals can also be used formetal oxide deposition. Especially in case of ozone, the substratesurface may remain too “dry” (i.e., the surface contains too few —OHgroups) and the number of active surface sites (especially —OH groups)will drop below an optimum value. By adding a certain amount of watervapor to the ozone pulse, the number of —OH groups on the surface can beincreased and the growth rate of the metal oxide thin film can beimproved. Alternatively, water vapor can be pulsed after the O₃ pulse toachieve the same result as with the water-ozone pulse.

Reduction Process

According to certain embodiments described herein, the metal oxide thinfilm that is to be reduced into a metal thin film consists essentiallyof a metal oxide or a mixture of metal oxides. The method of reducingthe metal oxide layer plays a very important role in certain embodimentsdescribed herein. The metal oxide is reduced in certain embodiments bymeans of gaseous inorganic reducing agents, such as thermal hydrogen(H₂), hydrogen plasma (H radicals), carbon monoxide (CO) or gaseousmixtures of said reducing agents optionally diluted with inactive gases,such as He. “Thermal hydrogen” means that hydrogen is in molecular form(H₂) and the hydrogen molecules have not been excited into radicals.Surprisingly, good adhesion of the reduced metal oxide thin film on thesubstrate is preserved when the above mentioned inorganic reducingagents are used under specified process conditions.

In certain embodiments, the conversion step is preferably done with areducing agent capable of forming a stronger bond to the oxygen of themetal oxide layer than the metal in the oxide layer. In certainpreferred embodiments, the reducing agent is in gaseous form. Thegaseous reducing agent is capable of taking away the oxygen that wasbound to the metal oxide and thus an elemental metal is left on thesubstrate surface. For example, hydrogen forms water (H₂O) molecules andcarbon monoxide forms carbon dioxide (CO₂) molecules.

The reduction process of preferred embodiments is carried out in areaction space that enables controlled temperature, pressure and gasflow conditions. The gaseous inorganic reducing agent is fed to thereaction space, optionally with the aid of an inactive carrier gas, suchas nitrogen, argon or helium. The reducing agent is contacted with thesubstrate, whereby the metal oxide layer is reduced at least partly to ametal layer and the reducing agent is oxidized. Typically, the reactionspace is then purged with an inactive carrier gas to remove theunreacted inorganic reducing agent and gaseous reaction byproducts.

The reduction process according to certain embodiments is preferablycarried out at low temperatures to avoid agglomeration of metal on thesubstrate surface. Theoretically, the reactions between oxide(s) and thereducing agents used in the process of certain embodiments are favorableover a wide temperature range, even as low as room temperature. Kineticfactors and the diffusion rate of oxygen and/or reducing agent in themetal oxide film set a lower limit on the actual process temperaturesthat can be applied economically to facilitate high enough throughput.The temperature in the reaction space is preferably in the range ofabout 50° C. to about 400° C., more preferably about 100° C. to about350° C. and even more preferably about 150° C. to about 300° C.

It is to be noted that in case of very thin metal oxide films, thereduction temperature can be selected from the lower side of thetemperature range. If the desired thickness of the metal oxide film ismore than about 50 nm and deposition and reduction temperature are verylow, thus causing a slow reduction reaction or slow diffusion of oxygenthrough the metal oxide layer, the deposition of the metal film can bedivided into at least two parts to speed up the total processing time.One layer of the metal oxide, comprising more than one molecularmonolayer of the metal oxide, preferably three or more monolayers, isdeposited by ALD, then reduced into a metal layer, another layer,comprising more than one molecular monolayer of the metal oxide andpreferably three or more monolayers, is deposited by ALD, then reducedinto a metal layer, et cetera, until a metal film of desired thicknessis obtained. Each thinner metal oxide layer is more susceptible toreduction than a single thick metal oxide layer.

The pressure in the reaction space is preferably 0.01 to about 50 mbar,more preferably 1 to 10 mbar during the deposition of the metal oxidefilm. During the reduction process, the pressure of the reaction spacecan be from about 0.1 mbar to over atmospheric pressure, more preferablyabout 0.5 to about 50 mbar, even more preferably about 3 to about 6mbar.

The processing time varies according to the thickness of the layer to bereduced and the reducing conditions, including, for example, the type ofreducing agent, the pressure in the reaction chamber and thetemperature. However, for layers having a thickness of approximately0.1-50 nm, the processing time is typically in the order of seconds.Layers thicker than about 50-100 μm can also be reduced in a batchprocess, as described above, to keep the reduction time per substrateshort enough. Preferably the layer to be reduced has a thickness of atleast about 0.6 nm, more preferably at least about 7 nm.

Suitable inorganic reducing agents are preferably selected from thegroup consisting of:

-   -   thermal hydrogen (H₂);    -   hydrogen radicals (H*); and    -   carbon monoxide (CO).

When hydrogen radicals are used for reducing the oxide film, thehydrogen radicals are generated via the formation of a hydrogen plasma.The plasma may be formed in situ, in the reaction chamber, or may beformed remotely and the radicals transported to the reaction chamber.The radicals are preferably generated in an atmosphere comprising about1% to about 30% flowing hydrogen, more preferably between about 3 andabout 10% flowing hydrogen and even more preferably about 3.85% flowinghydrogen. The atmosphere also preferably comprises one or more inertgases, such as He. For example, a plasma may be produced in anatmosphere comprising 3.85% hydrogen in helium by applying power to aflowing gas comprising 10 sccm H₂ and 250 sccm He.

The pressure in the reaction chamber for the generation of hydrogenradicals is preferably between about 1 and 10 mbar, more preferablybetween about 3 and 6 mbar, and even more preferably about 5.6 mbar.

A plasma power setting of about 200 to about 2000 W is typically used togenerate the hydrogen radicals. In a preferred embodiment, a powersetting of about 1500 W is used.

In a particularly preferred embodiment, oxide reduction is carried outusing hydrogen radicals generated using a 1500 W plasma at a pressure of5.6 mbar and a temperature of about 150° C. in an atmosphere comprisingabout 3.85% hydrogen in helium.

Reactors used for deposition of thin films by ALD and/or CVD arepreferably used in the methods of certain embodiments described herein.However, the deposition of the metal oxide thin film and the reductionstep in embodiments using inorganic reduction agents are preferablycarried out sequentially in one reactor. The reduction process can alsobe done in a cluster tool where the substrate arrives from a previousprocess step, the substrate is treated with the reducing agent and thentransported to the following process step. In a cluster tool thereaction space temperature of each chamber can be kept constant, whichimproves the throughput when compared to a reactor which is heated tothe process temperature before each run.

With reference to FIG. 4, typically, the processes include cleaning 400of a substrate that is for example a 200-mm or 300-mm silicon wafer,deposition 402 of a diffusion barrier layer on the substrate, deposition404 of a metal oxide layer on the diffusion barrier layer, reduction 406of the metal oxide layer into a metal layer and deposition 408 of bulkmetal on the metal layer.

In an exemplary embodiment, a conformal copper oxide (CuO) film isdeposited by ALD on a diffusion barrier film. Suitable diffusion barrierfilms for copper include, but are not limited to, TiN, Ta, TaN and WNC(tungsten nitride carbide). The CuO film has a thickness preferably atleast about 0.6 nanometers, more preferably between approximately 1nanometer and approximately 20 nanometers, and most preferably betweenapproximately 1 nanometer and approximately 3 nanometers.

After a reduction process that removes oxygen from the copper oxidelayer, the resultant Cu film can then be used as a seed layer in an ECDtool for subsequent metal layer formation. In certain embodiments, theseed layer has a resistivity of preferably between about 1 μΩ-cm andabout 30 μl-cm, more preferably between about 1.67 μΩ-cm and about 10μΩ-cm, and most preferably between about 1.7 μΩ-cm and about 3 μΩ-cm.Resulting structures can be used in microchip metallization such assingle and dual damascene processes.

In certain preferred embodiments, the subsequent metal layer formationis performed using an ECD tool. An exemplary ECD tool comprises at leastone ECD module. An example of ECD tools is LuminaCu™ system availablefrom NuTool, Milpitas, Calif., USA. In addition, prior to the formationof the subsequent metal layer using ECD, the resulting Cu seed layer canbe repaired using known seed repair technologies, such as electrolessdeposition processes (see, e.g., Peter Singer, “Progress in Copper: ALook Ahead,” Semiconductor International, May 1, 2002, the disclosure ofwhich is incorporated in its entirety by reference herein). The basicsof electroless deposition, also known as electroless plating, have beenpresented by G. Mallory and J. Hadju in “Electroless Plating:Fundamentals and Applications”, Noyes Publications, 1990, which is alsoincorporated in its entirety by reference herein.

Nickel oxide (NiO), silver oxide (AgO), cobalt oxide (CoO), palladiumoxide (PdO) and ruthenium oxide (RuO₂) serve as examples of other metaloxides that can be reduced with the present method into metal layers tobe used as seed layers for ECD.

A surprising finding related to certain embodiments described herein isthat the film has very good adhesion to the substrate, even after areduction step. The structural integrity of the metal film is preservedand the formation of pinholes in the film is avoided. While the exactnature of the interface between the metal film and the substrate isunclear, it is obvious that the interface is much stronger than in thecase of direct deposition of metal films by ALD.

EXAMPLES Example 1 ALD of Cobalt Oxide

Co(thd)₃ and O₃ were used as source chemicals for the cobalt oxidedeposition in an ALD reactor. Co(thd)₃ was heated to 110° C. O₃ wasprepared from 99.9999% O₂ with an external ozone generator. Theresulting oxygen source gas mixture consisted of 10-20 vol.- % O₃ in O₂.Nitrogen, evaporated from liquid nitrogen, was used as an inert purginggas. Co(thd)₃ pulse length varied from 1.5 s to 2.0 s, while O₃ pulse(flow rate 100 std.cm³/min) length varied from 2.0 s to 4.0 s. Siliconwas used as the substrate material. Substrate temperatures between 150°C. and 350° C. were tested. One pulsing cycle consisted of foursequential steps: Co(thd)₃ pulse, N₂ purge, O₃ pulse, N₂ purge.

The higher deposition temperature tested resulted in uncontrolled filmgrowth, as Co(thd)₃ decomposed thermally thus producing a poor thicknessprofile for the thin film. At the lower substrate temperatures, acontrolled growth rate of the thin film (0.3 Å/cycle) and good adhesionwere obtained. A total of 2000 pulsing cycles resulted in a 64 nm thickcobalt oxide layer. According to Energy Dispersive X-ray Spectroscopy(EDS) measurements the thin films consisted of CoO.

Example 2 ALD of Palladium Oxide

Substrates with Si, TiN, WN, W₃C and SiO₂ surfaces were loaded into anF-120 ALD reactor manufactured by ASM Microchemistry Ltd., Finland.Pd(thd)₃ was loaded into a solid source tube of the reactor. The reactorwas pumped to vacuum. The pressure of the reaction chamber was adjustedto about 5-10 mbar with flowing nitrogen gas while the pumping of thereactor continued. The Pd(thd)₃ was heated to 110° C. and the reactionchamber to 150° C.

One pulsing cycle consisted of four steps in the following order:Pd(thd)₃ pulse (2.0 s), N₂ purge (1.0 s), O₃ pulse (4.0 s), N₂ purge(2.0 s).

The growth rate of palladium oxide from Pd(thd)₃ and O₃ was 0.15 Å/cycleat 150° C. According to EDS the film consisted of palladium oxide. Thefilm grew on Si, TiN, WN, WXC (tungsten carbide) and SiO₂ surfaces andshowed good adhesion.

Example 3 Thermal Hydrogen as a Reducing Agent for the Reduction ofALD-Grown Copper Oxide

A silicon substrate having 20 nm of thermal silicon dioxide on thesurface was loaded to the reaction space of a Pulsar® 2000 ALCVD™reactor. The pressure of the reaction space was adjusted to about 1-20mbar with a vacuum pump and flowing nitrogen gas (claimed purity99.9999%). The temperature of the reaction space was adjusted to about300-315° C. Tungsten nitride carbide (WNC) thin film was deposited onthe thermal SiO₂ from alternate pulses of WF₆, NH₃ and triethylboron(TEB). The deposition cycle consisted of WF₆ pulse 0.25 s, N₂ purge 1 s,NH₃ pulse 0.75 s, N₂ purge 1 s, TEB pulse 0.1 s and N₂ purge 1 s. Thesepulse and purge times serve as examples of suitable values for thedeposition process. Typically, pulse and purge times are selected from arange of about 0.05 s-3 s. Depending on the experiment, the depositioncycle was repeated 30-150 times, resulting in a WNC thin film having athickness of 25-120 angstroms. Details of the WNC deposition at lowtemperatures have been disclosed in U.S. patent application publicationno. 2003/0082296, the disclosure of which is incorporated herein byreference.

Next, the substrate was transferred to another Pulsar® 2000 ALCVD™reactor. The pressure of the reaction space was adjusted to about 1-20mbar with a vacuum pump and flowing nitrogen gas (claimed purity99.9999%). The temperature of the reaction space was selected from arange of about 110-140° C. Copper oxide (CuO) thin film was deposited onthe WNC film from alternate pulses of Cu(acac)₂ and ozone (O₃). Ozonewas formed from oxygen gas in an external oxygen generator. The flowrate of the O₃/O₂ mixture was set to 200 std.cm³/min (200 sccm). Therewas about 15% of O₃ in O₂. Cu(acac)₂ was heated to a source temperaturethat was selected from a range of about 110-140° C. The deposition cycleconsisted of Cu(acac)₂ pulse 0.1-2 s, N₂ purge 0.05-1 S, O₃ pulse 0.1-1s and N₂ purge 1-3 s. These pulse and purge times serve as examples ofsuitable values for the deposition process. Typically, pulse and purgetimes are selected from a range of about 0.05 s-7 s. Depending on theexperiment, the deposition cycle was repeated 5-1500 times, resulting ina CuO thin film having a thickness of about 3-350 angstroms.

Then the substrate was transferred to an Eagle® 12 reactor, commerciallyavailable from ASM Japan K.K. of Tokyo, Japan. The temperature of thereaction space was set to a value selected from a range of about270-320° C. Lower temperatures help to avoid the agglomeration of coppermetal during the reduction process. The reaction chamber was purged withnitrogen gas at lowered pressure and then the substrate was exposed to500 sccm (std. cm³/minute) of thermal H₂ for about 22 s for reducing thecopper oxide film. As a result, smooth copper thin film was obtained onthe substrate. Depending on the experiment and desired metal thickness,up to about 10 nm of Cu had excellent adhesion on the WNC. The samplespassed a Scotch™ tape test. ESCA depth profiling confirmed that thecopper oxide films had successfully been reduced into metallic copperfilm and very little oxygen was seen at the WNC/Cu interface.

Some of the copper seed samples were coated with bulk copper in an ECDtool and some of the samples received bulk copper coating in an MOCVDreactor. Bulk copper metal had excellent adhesion to the substrates.

Example 4 Reduction of Copper Oxide Using Hydrogen Plasma as a reducingAgent

After depositing WNC on thermal SiO₂ and CuO on the WNC, the 200 mmsilicon wafer was transferred to an Eagle® 10 reactor, commerciallyavailable from ASM Japan K.K. of Tokyo, Japan, for reducing the copperoxide film with direct plasma. In one experiment 35 nm of CuO wasreduced into copper film at 300° C. in 4 seconds. In other experiments35 nm of CuO was reduced with hydrogen plasma at 150° C. in 15 s and 17nm of CuO was reduced into copper metal with hydrogen plasma at 150° C.in 10 s. The resistivity of the resulting Cu film was lower than 10μΩ-cm. Very uniform sheet resistance was obtained and the variation wasonly 2.2% over a 200 mm wafer. The film contained less than 50 ppm ofFe, Cr and Ni impurities. It is also possible to apply remote (asopposed to direct or in situ) plasma for the reduction of metal oxides,including copper oxide.

Two additional sets of experiments were performed on wafers prepared asdescribed above.

One set of experiments was done using a 1500 W plasma at a pressure of560 Pa and a temperature of 150° C. The set-up of these experiments canbe seen in the first half of Table 1. 1000 (˜35 nm) and 500 (˜17 nm)cycle Cu_(x)O layers were reduced in a 20, 10 and 3.85% H₂ atmosphere.1500 cycle (˜30 nm, the difference in thickness could be explained bythe fact that these layers were deposited at a different time underdifferent conditions) Cu_(x)O layers were reduced in a 1 and 2% H₂atmosphere and finally a 200 cycle (˜7 mm) Cu_(x)O layer was reduced ina 3.85% H₂ atmosphere.

The second set of experiments was done using a 250 W plasma at apressure of 300 Pa in a 3.85% H₂ atmosphere. The setup of theseexperiments can be seen in the first half of Table 2. First some trialexperiments were done exposing 1000 cycle Cu_(x)O layers for a shorttime at 150, 200 and 300° C. After this some more extensive experimentswere done on 1000 cycle Cu_(x)O layers at 200° C. and 150° C. Also someexperiments at 150° C. were done ori 200 and 500 cycle Cu_(x)O layers.

Rs measurements were made on all wafers. In addition, the compositionsof a non-reduced, a half-reduced (Table 2 no. 6) and a fully reducedlayer (Table 1 no. 9) were analyzed.

All wafers had a light blue or silver color prior to reduction. The Rsmeasured d before reduction was most likely that of the WNC, since thevalues agreed and the Rs of an oxide is too high to measure. For allwafers this value was more or less the same and showed a similar patternacross the wafer.

After reduction all wafers also showed more or less the same Rs patternacross the wafer. TABLE 1 Experimental Set-up and Results for PE H₂Reductions at 1500 W, 560 Pa and 150° C. Before After Wafer CuO H₂/HeTime Rs Rs No. [cycles] [sccm] H₂ [%] [sec] [Ω/Sq] SD [%] [Ω/Sq] SD [%]Color (after) Agglomeration 1 1000 20/80 20 5 464 1.22 7.26 9.88 blue —2 1000 20/80 20 5 407 2.94 8.15 5.72 copper — 3 1000 20/80 20 10  4921.72 4.09 4.65 blue — 4 1000 20/80 20 15  467 1.25 3.95 4.67 bluepartial 5 1000  20/180 10 5 466 1.48 4.89 3.36 blue partial 6 1000 10/9010 5 413 1.23 3.63 2.27 copper partial 7 1000  10/250 3.85 5 387 2.093.71 4.05 copper no 8 1500  7/353 2 5 419 0.687 5.24 18.4 purple spot —9 1500  7/343 2 +5* 5.24 18.4 3.01 3.69 copper holes 10 1500  3.5/346.51 5 399 0.957 —** — copper — 11 500 20/80 20 5 344 0.594 21.8 5.54silver — 12 500 10/90 10 5 427 1.44 25.4 4 silver — 13 500  10/250 3.855 438 3.24 14.8 5.72 silver partial 14 200  10/250 3.85 5 374 1.58 37418.4 silver full— No measurement or examination was done*After reducing for 5 sec. the wafer was reduced for another 5 sec.**No Rs was measured, because the wafer was partially delaminated at theedge

Table 1 shows the results of the reductions done with a plasma power of1500 W at 560 Pa.

For 1000 cycle Cu_(x)O layers a 10/90 sccm H₂/He-flow gave a betterresult than the 20/180 sccm H₂/He-flow settings at 10% H₂. Thedifference in Rs before reduction is not believed to be of anyinfluence, since this value is solely due to the WNC barrier.

The 1500 cycle Cu_(x)O layer had a significantly lower Rs afterreduction, which is in accordance with a thicker Cu_(x)O layerassociated with the higher number of deposition cycles. However for the1500 cycle layer deposited on a thermal oxide monitor wafer, a thicknessof 30 nm was measured, while for the 1000 cycle layer a thickness of 35nm was measured. This suggests that the 1500 cycle Cu_(x)O layer isthinner than the 1000 cycle layer, which is in contradiction with thelower Rs after reduction. These results indicate that the 1500 cycleCu_(x)O layer is differs from the 1000 cycle layers as a result ofdifferent deposition conditions.

The Rs of the 1000 and 1500 cycles is plotted against the reduction timein FIG. 7, providing an indication of the Rs after reduction and thetime necessary for complete reduction of the thicker Cu_(x)O layers atdifferent H₂ settings. It can be seen that for all settings the Rs afterreduction is more or less the same: ˜4 Ω/Sq. For the 2 and 20% H₂ ittook a bit more time to fully reduce the wafer.

The 1000 cycle Cu_(x)O wafer reduced with 3.85% H₂ showed no significantdamage.

The 500 cycle Cu_(x)O layers were all of the same lot, minimizinginter-wafer differences. In FIG. 8 the Rs of the reduced wafers isplotted against the percentage of H₂. The 3.85% H₂ gave the best resultof ˜15 ΩSq, followed by the 20% H₂ and the 10% H₂ gave the highest Rs.

For the 200 cycle Cu_(x)O layer no change in Rs was measured. However,SEM pictures indicated that the layer was completely agglomerated. Thissuggests that the layer was both reduced and agglomerated at the sametime. TABLE 2 Experimental Set-up and Results for PE H₂ Reductions at250 W, 300 Pa and 3.85% H₂ Before After CuO Time Rs Rs No. [cycles] Temp[° C.] [sec] [Ω/Sq] SD [%] [Ω/Sq] SD [%] Color (after) Agglomeration 11000 150 10 464 1.37 464 1.38 yellow spot — 2 1000 300 10 474 1.68 5776.66 yellow spot — 3 1000 200 10  160* 0.989 160 1.03 yellow spot Holes4 1000 200 30  160* 1.01 90.9 88.0 yellow spot — 5 1000 200  +30**  90.9 88.0 4.13 6.14 silver Holes 6 1000 200 30 517 5.25 198 110 purplespot Holes 7 1000 200 45 415 1.10 3.74 3.04 copper — 8 1000 150 45 3982.12 3.75 4.34 copper Holes 9 500 150 15 386 0.720 389 5.53 brown spot —10 500 150 25 398 0.963 24.7 3.64 silver Partial 11 500 150 35 510 5.6424.9 1.06 silver — 12 200 150 15 456 2.40 454 2.39 silver — 13 200 15025 392 1.16 390 1.17 silver —— No examination was done*300 cycles WNC instead of 150 cycles**After reducing for 30 sec. the wafer was reduced for another 30 sec.

Table 2 shows the results of reduction of Cu_(x)O layers with a plasmapower of 250 at 300 Pa. Here, as in the experiments with 1500 W power,different wafer lots resulted a different color after reduction. Allhalf-reduced wafers showed a colored spot in the middle of the wafer,while the edge already had the color of the fully reduced wafer. Onlythe wafer of experiment 2 had a dark blue color on the edge. Experimentsno. 3 and 4, in which wafers of the same lot were used, showed a lowerRs before reduction. The Rs found for these wafers, is in accordancewith a 300 cycle WNC layer.

The first experiments consisted of exposing a 1000 cycle Cu_(x)O layerfor a short time at 150, 200 and 300° C. These experiments showed that10 seconds was too short to reduce the 1000 cycle Cu_(x)O layer. Forthis reason some more extensive experiments were done, in which a 1000cycle Cu_(x)O layer was exposed to the reducing agent for a longer timeat 200° C. and once at 150° C.

In FIG. 9, the Rs of the 1000 cycle Cu_(x)O layers is plotted againstthe reduction time. It can be seen that, for 200° C. it takes about 45seconds to fully reduce the layer and reach a value of ˜4 Ω/sq. After 30seconds a wide spread across the wafer can be seen, this is reflected inthe standard deviation. The same increase in Rs was seen after a 10second reduction at 300° C.

The wafer reduced at a temperature of 150° C. also seemed to be reducedin 45 seconds, because an Rs of ˜4 Ω/sq. was reached. However the Rsplot still showed a somewhat circular pattern, which indicates that theCu_(x)O was not completely reduced yet.

FIG. 10 shows the Rs plotted against the reduction time for thedifferent cycle numbers of the Cu_(x)O deposition. The reactiontemperature was 150° C. There is a clear dependence on Cu_(x)O layerthickness. For the 500 cycle Cu_(x)O layer it takes 25 seconds to fullyreduce the layer and reach an Rs of ˜26 Ω/sq. where it levels off. Thishigher Rs is probably caused by both an increased electronsurface-scattering and partial agglomeration. The Rs of the 200 cycleCu_(x)O layer hardly changes. This is probably caused by totalagglomeration of the Cu layer so no current path through the copper isavailable anymore and the only thing that can be measured is the sheetresistance of the underlying barrier.

For plasma enhanced H₂ reduction of ˜35 nm Cu_(x)O layers no significantdifferences were found in Rs for different percentages of H₂ ordifferent power settings at 150° C. For all settings an Rs of 4 Ω/sq.was reached. The best result was obtained with a 1500 W plasma at 560 Paand 3.85% H₂. This was the only layer that did not show any significantdamage. However, a big difference in reduction time was found betweenthe different plasma settings. For the 1500 W plasma at 450 Pa, 5 to 10seconds were needed to fully reduce a layer. On the other hand, for the250 W plasma at 300 Pa it took slightly more than 45 seconds.

For ˜17 nm Cu_(x)O layers, differences were seen at 150° C. fordifferent percentages H₂ and different power settings. The best resultwas again obtained with a 1500 W plasma at 560 Pa and 3.85% H₂. Thereduced layer showed small holes and the start of agglomeration, butthis did not seem to influence the resistance much, as it was only ˜15Ω/sq. The 250 W plasma at 300 Pa and 3.85% H₂ and the 1500 W at 560 Paand 10% H₂ preformed the worst with an Rs of ˜25 Ω/sq. It was notpossible to reduce ˜7 nm Cu_(x)O layers without complete agglomeration.

Analyses showed that the Cu_(x)O layer was a mixture of CuO and Cu₂O. Italso showed that the plasma enhanced H₂ reduction is a 3-step top-downprocess. First all CuO is reduced to Cu₂O. This is then reduced to amixture of Cu₂O and Cu, which is finally reduced to pure Cu. During thisprocess a thickness reduction of over 50% is observed.

A comparison between the reduction results obtained with thermalhydrogen and hydrogen plasma shows that the reduction with hydrogenplasma was much faster and could be done at a lower process temperaturethan with thermal hydrogen.

Example 5 Processing in a Cluster Tool Including Reduction of CopperOxide Using Hydrogen Plasma as a Reducing Agent

FIG. 5 schematically illustrates a first cluster tool 500 in accordancewith embodiments described herein. The substrate is cleaned (e.g.,sputter-cleaned using nitrogen, ammonia, or argon plasma) in a firstreaction chamber (i.e., process module) 530 of the cluster tool 500. Thesubstrate is then moved to a second reaction chamber 540 of the clustertool 500, in which a diffusion barrier layer, for example tungstennitride carbide (WNC), is deposited by ALD on the substrate. Thethickness of WNC can be selected, e.g., from the range of 1-6 nm. Thefirst cluster tool 500 further comprises a vacuum transport module 520and one or more load locks 510, 550 to transfer the substrate to andfrom the cluster tool 500.

A barrier processing sequence in which the diffusion barrier isdeposited before sputter-cleaning of the vias is also possible. Thebenefit of this type of processing sequence is that copper removed fromthe via bottom during the cleaning step cannot contaminate the via wallsbecause the sidewalls are covered with the copper diffusion barrier asthe Cu at the bottom of the via is cleaned by the directional etch.

FIG. 6 schematically illustrates a second cluster tool 600 in accordancewith embodiments described herein. The substrate is transported from thefirst cluster tool 500 (FIG. 5) to the next processing unit, e.g., asecond cluster tool 600. In this embodiment the substrate is moved to afirst reaction chamber 630 of the second cluster tool 600 in whichcopper oxide (CuO and/or Cu₂O) is deposited by ALD on the diffusionbarrier surface (e.g., WNC). The thickness of copper oxide can beselected, e.g., from a range of 0.6-10 nm. After that the substrate ismoved to a second reaction chamber 640 of the second cluster tool 600and copper oxide is reduced into copper metal, for example with thermalH₂, hydrogen radicals or carbon monoxide. After the reduction processthe substrate is moved to a third reaction chamber 650 of the secondcluster tool 600 and an MOCVD or copper superfill process is used fordepositing bulk copper metal on the copper seed layer. The secondcluster tool 600 further comprises one or more load locks 610, 660 and avacuum transport module 620. According to another embodiment thesubstrate is transported from the second reaction chamber 640 to an ECDtool (not shown) for depositing bulk copper metal on the copper seedlayer so that vias and trenches are filled with the bulk copper metal.Advantageously, ECD tools are typically less costly than an MOCVD bulkcopper cluster tool.

In the ECD tool the substrate is placed in a electroplating solutionthat contains a water-soluble copper compound, some acid to lower the pHof the solution, and standard additives that are commonly used toimprove the quality of the growing copper film. A voltage is appliedbetween the substrate and an opposing electrode. Copper is depositedfrom the solution on the seed layer, and vias and trenches become filledwith copper metal. The opposing electrode consists of pure copper thatdissolves into the electroplating solution during the electroplatingprocess. After the electroplating process, the substrate is rinsed toremove residual electroplating solution. The substrate is then ready forchemical mechanical polishing (CMP).

Various embodiments of the present invention have been described above.Although this invention has been described with reference to thesespecific embodiments, the descriptions are intended to be illustrativeof the invention and are not intended to be limiting. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention asdefined in the appended claims.

1. A method of producing a conductive thin film, comprising: depositinga metal oxide thin film on a substrate by an atomic layer deposition(ALD) process; and at least partially reducing the metal oxide thin filmby exposing the metal oxide thin film to a gaseous inorganic reducingagent, thereby forming a metal layer.
 2. The method of claim 1, whereinthe metal oxide thin film is at least 0.6 nanometers thick.
 3. Themethod of claim 1, wherein the metal oxide thin film has a thicknessbetween approximately 1 nanometer and approximately 20 nanometers. 4.The method of claim 1, wherein the metal oxide thin film has a thicknessbetween approximately 1 nanometer and approximately 3 nanometers.
 5. Themethod of claim 1, wherein depositing comprises at least three cycles ofthe ALD process.
 6. The method of claim 1, wherein the metal oxide thinfilm is selected from the group consisting of ReO₂, Re₂O₅, ReO₃, RuO₂,OSO₂, CoO, CO₃O₄, Rh₂O₃, RhO₂, IrO₂, NiO, PdO, PtO₂, Cu₂O, CuO, AgO,Ag₂O, and Au₂O₃.
 7. The method of claim 1, wherein the ALD processcomprises feeding into the reaction chamber and contacting the substratewith alternating vapor phase pulses of at least one first sourcechemical comprising a compound capable of adsorbing no more than amolecular monolayer of metal species on the substrate and at least onesecond source chemical comprising a compound capable of oxidizing themetal species on the substrate into the metal oxide.
 8. The method ofclaim 7, wherein the first source chemical is Cu(thd)₂ and the secondsource chemical is selected from the group consisting of ozone (O₃),oxygen (O₂) and a mixture of O₃ and O₂.
 9. The method of claim 7,wherein the first source chemical is copper(II)acetylacetonate Cu(acac)₂and the second source chemical is selected from the group consisting ofozone (O₃), oxygen (O₂) and a mixture of O₃ and O₂.
 10. The method ofclaim 7, wherein the first source chemical is Co(thd)₃ and the secondsource chemical is selected from the group consisting of ozone (O₃),oxygen (O₂), and a mixture of O₃ and O₂.
 11. The method of claim 7,wherein the first source chemical is Pd(thd)₃ and the second sourcechemical is selected from the group consisting of ozone (03), oxygen(O₂), and a mixture of O₃ and O₂.
 12. The method of claim 1, wherein thesubstrate comprises a barrier film and the metal oxide thin film isdeposited onto the barrier film.
 13. The method of claim 12, wherein thebarrier film comprises a material selected from the group consisting ofTiN, Ta, TaN and WNC.
 14. The method of claim 1, wherein the inorganicreducing agent is selected from the group consisting of thermal hydrogen(H₂), hydrogen radicals (H*) and carbon monoxide (CO).
 15. The method ofclaim 1, further comprising depositing metal onto the metal layer by anelectrochemical deposition (ECD) process in an ECD tool.
 16. The methodof claim 15, further comprising repairing the metal layer prior to theECD process.
 17. The method of claim 1, further comprising depositingmetal onto the metal layer by a metal organic chemical vapor deposition(MOCVD) process in a CVD tool.
 18. The method of claim 1, furthercomprising depositing metal onto the metal layer by a copper superfillprocess in the CVD tool.
 19. The method of claim 1, wherein reducing themetal oxide thin film essentially converts the metal oxide into anelemental metal seed layer which has sufficient electrical conductivityto be used for subsequent electrochemical deposition.
 20. The method ofclaim 1, wherein reducing the metal oxide thin film essentially convertsthe metal oxide into an elemental metal to provide sufficient electricalconductivity to be used as an electrode of a capacitor.
 21. The methodof claim 1, wherein the metal layer has a resistivity between about 1μΩ-cm and about 30 μΩ-cm.
 22. The method of claim 1, wherein the metallayer has a resistivity between about 1.67 μΩ-cm and about 10 μl-cm. 23.The method of claim 1, wherein the metal layer has a resistivity betweenabout 1.7 μΩ-cm and about 3 μΩ-cm.
 24. A method of producing aconductive thin film comprising the steps of: A. placing a substrate ina chamber; B. exposing the substrate to a vapor phase first reactant,wherein the first reactant adsorbs no more than a molecular monolayer ofmetal species on the substrate; C. removing excess first reactant andgaseous reaction byproducts from the chamber; D. exposing the substrateto a second vapor phase reactant comprising a compound that is capableof oxidizing the adsorbed metal species on the substrate into metaloxide; E. removing excess second reactant and gaseous reactionbyproducts from the chamber; F. repeating the above steps B through E atleast three times to form a metal oxide film; and G. following step F,exposing the substrate to a gaseous inorganic reducing agent to reducethe metal oxide film to metal.
 25. The method of claim 24, wherein instep F, is repeated at least 10 times to form the metal oxide film priorto step G.
 26. A method of producing an electrically conductive thinfilm, comprising: depositing a metal oxide thin film on a partiallyfabricated integrated circuit by an atomic layer deposition (ALD)process, the metal oxide thin film having a thickness of at least 0.6nm; and at least partially reducing the metal oxide thin film toelemental metal with a gaseous inorganic reducing agent.
 27. The methodof claim 26, wherein depositing the metal oxide thin film comprises anatomic layer deposition process.
 28. The method of claim 27, wherein atleast partially reducing the metal oxide thin film comprises exposingthe metal oxide thin film to one or more gaseous inorganic compoundsselected from the group consisting of thermal hydrogen (H₂), hydrogenradicals (H*) and carbon monoxide (CO).
 29. The method of claim 28,wherein the reduction temperature is between about 80° C. and about 350°C.
 30. The method of claim 29, wherein the reduction temperature isbetween about 150° C. and about 300° C.
 31. The method of claim 30,wherein the reduction temperature is between about 200° C. and about300° C.
 32. The method of claim 26, wherein the metal layer is used as aseed layer for a subsequent metal deposition.
 33. The method of claim 28wherein partially reducing the metal oxide thin film comprises exposingthe metal oxide thin film to hydrogen radicals.
 34. The method of claim33, wherein the hydrogen radicals are generated in an atmospherecomprising between about 3 and about 10% flowing hydrogen.
 35. Themethod of claim 34, wherein the hydrogen radicals are generated in anatmosphere comprising about 3.85% flowing hydrogen.
 36. The method ofclaim 33, wherein the hydrogen radicals are generated at a plasma powersetting of about 200 to about 2000 W.
 37. The method of claim 36,wherein the hydrogen radicals are generated at a plasma power setting ofabout 1500 W.
 38. The method of claim 33, wherein the hydrogen radicalsare generated by applying a power of 1500 W to an atmosphere comprisingabout 3.85% flowing hydrogen.